1. Field of the invention
The present invention relates to the conduction of light in thin layers, and more in particular to components with suchlike layers comprising a medium with electro-optical (e/o) properties with which under the influence of an electric field changes in the refractive index are induced in the medium in such a way that as a consequence of that the conduction of light in the thin layers will be influenced.
Moreover the present invention comprises a method for making components of the above-mentioned type.
The invention is especially applied in the field of integrated optical techniques.
2. State of the art
Components of the aforesaid type are known per se, for example from the references (1), (2), (3) and (4) mentioned under D. They comprise a substantially plane light conducting layer of electro-optically active material, and are moreover provided with at least two cooperating electrodes disposed on one side or on both sides on said layer in patterns preselected, and possibly kept separated from the electro-optically active layer by light insulating or buffer material--which is not electro-optically active and with a refractive index which is equal to or lower than the electro-optically active material in a non-activated state. Owing to a difference in voltage applied an electric field will be generated between said electrodes, which field will, as long as it exists, cause in the electro-optical material a spatial refractive index profile or a change in it dependent on the direction and the extent of uniformity of the electric field induced between these electrodes, and on the direction of the optical axis of the material Because of the fact that the working of the electric field induces in the e/o material a spatial refractive index profile, said profile will be strongly dependent on the electrode patterns chosen. Known are straight or bent usually strip-shaped electrodes, of which at least two of them either parallel next to each other, as in (2), or one over the other, as in (1) work together, i.e. across which the required difference in voltage for the desired electric field is applied in the desired direction. Moreover it is known from reference (1) and reference (3) that the strip-shaped electrodes on the one (upper) side of the plane layer of e/o material work together with an electrode extending over the whole plane layer on the other (under) side of said layer.
The spatial refractive index profiles induced by means of electrode patterns of the aforesaid type are usable as light waveguides in the plane light conducting layer, to which a connection of permanent light waveguides such as for example glass fibers is feasible. As to their nature these electro-optically inducible light waveguides can be switched on and off. The refractive index profiles for the desired light waveguide patterns obtained by the electrode patterns known from the cited literature have a somewhat tube-shaped character with often more or less elliptical cross sections. A cross-sectional view shows that a change in the refractive index is greatest in the center area and away from it decreasing, either gradually to the level of the refractive index of the e/o material, on which the inducing electric field has no or hardly any influence as yet, or at a leap to the level of the refractive index of the contiguous non-e/o buffer material.
The light retaining quality of the induced light conductor, particularly with regard to losses of light energy is dependent on the quality of the lateral definition, with which is indicated the degree of steepness of the decrease, or the leap in the course of the refractive index, towards the level of the refractive index of the surrounding material seen in cross section from the center area of the light conductor. The lateral definition is determined either by the electric field itself or by the bounding surfaces of the e/o material contiguous to the non-e/o material. With regard to this definition the bounding surfaces at the top and at the bottom will usually not cause any difficulties, certainly not in the case of thin layers with thicknesses to 10 .mu.m. The electric fields obtained by means of co-operating electrode patterns on both sides of a plane layer of e/o material of a thickness of said order of magnitude are immediately, on both sides of said bounding surfaces, such that a sufficiently great refractive index leap will be induced. The lateral definition on both sides of the induced light waveguide in the plane layer itself will, however, be determined exclusively by the course of the electric field strength and its decreasing uniformity. In that case the decrease of the strength of the field away from the center area of the light conductor will always have a certain graduality and this does not make it possible to achieve a sharp refractive index transition. A better lateral bounding can be obtained by laterally bounding the e/o material with light insulating material. This is aimed at in the above-cited technique, such as disclosed in references (2) and (3), by placing an extra elongated ridge of e/o material integrated with the plane layer underneath it, in which case, however, part of the bounding is still determined exclusively by the electric course of the field. Besides similar lateral boundings of the e/o material are obtainable by means of otherwise known photolithographic techniques, in which case the e/o material is applied as a layer after which that part of the material which is not usable for the light conductor(s) is etched away. Apart from the fact that this involves extra complicating processes, the side planes obtained by said etching away still have a considerable amount of wall roughness with regard to the conduction of light.
As a solution to said problem, which is even greater in the outside curve of a bend of a ridge-shaped light conductor, reference (4) proposes to cover etched-sidewall ridge-shaped light conductors with a special upper layer of heat resistant synthetic resin of an accurately determined thickness, with which the optical losses in consequence of wall roughness will be reduced. Apart from the fact that it remains to be seen whether due to the above solution in most cases a sufficient reduction of said losses can be achieved, it will remain laborious to utilize that solution.
Reference (1) further discloses an e/o component in which a light waveguide can be induced as it were between two more or less reflecting vertical parallel "walls" at a short distance from each other in the thin layer of e/o material. These reflecting "walls" can be induced by electric fields generated between two pairs of strip-shaped co-operating electrodes disposed on both sides of the e/o material, which fields can effect a decrease of the refractive index in the e/o material between each of the pairs of electrodes. The e/o area between the "walls" will not or hardly be influenced by the fields, and has then to serve as a light waveguide. The lateral definition of such a waveguide will here be more or less complementary to the one of the above-described cases and consequently even less good from a qualitative point of view.
From reference (3) it is further known that the e/o material in the area where two electrodes disposed on both sides of the plane layer work together is locally poled. This means that the e/o material in that area got its optical orientation when the component was made. This takes place by bringing the poleable glassy polymer, of which the e/o material consists, under the influence of an external electric field, when it is still in a state above the glass softening temperature. The electric field may be generated via said co-operating electrodes Said optical orientation has subsequently been frozen as it were by cooling it down. Outside said area the material is isotropic and electro-optically inactive, but here too the lateral definition of the light waveguide induced in that area cannot be better than the one determined by either the course of the instantaneous inducing electric field or by the course of the degree of ("frozen") e/o activity in consequence of the field under the inducing influence of which the material is poled; all this depends on the degree to which the instantaneous field corresponds to the field with which has been poled.
Summarizing, it can be stated that, in this known technique in the field of the e/o influencing of the conduction of light in thin layers electro-optically induced refractive index transitions, especially those which serve for a total reflection, as used in the case of the temporary establishment of light waveguides, the lateral definition is substantially determined either by the inducing field or by bounding surfaces contiguous to non-e/o buffer material, which bounding surfaces, obtained e.g. by means of known lithographic techniques, have a relatively great wall roughness. This can in both cases lead to unwanted losses of light energy in the e/o components, in the second case the making of them is moreover extra complicated.